1. Field of Invention
The present invention relates to a membrane separation device using activated sludge process suitable for water treatment, such as sewage treatment and wastewater treatment.
2. Description of the Related Art
Conventionally, in sewage treatment and wastewater treatment employing the activated sludge process, a membrane separation device of submerge type is used for solid-liquid separation which is necessary in the treatment process.
International Patent Publication No. WO2009/118785 discloses a pair of liquid collection unit, a plurality of membrane elements which are arranged between the pair of liquid collection units, and a membrane module having such a structure that filtered liquid which has passed through the membrane elements is collected in the liquid collection units.
The membrane modules are stacked in multiple stages and connected to the liquid collection units which are arranged adjacent thereto in a vertical direction such that filtered liquid is flow through between the liquid collection units.
A frame body accommodating the stacked membrane modules having multiple stages is used in order to maintain the orientation and connection state of each of the membrane modules and integrally install the membrane modules when the stacked membrane modules having the multiple stages are submerged into a biological treatment tank and the like and solid-liquid separation operations are performed.
It is preferable that the membrane modules, including the liquid collection units, are made of a resin to reduce the weight, while the frame body is made of a metal in many cases so as to provide a sufficient strength so as to hold each of the membrane modules in a stable position when it is disposed and submerged in the biological treatment tank and the like, as well to allow the membrane modules accommodated in the frame body to be integrally submerged into or taken out from the biological treatment tank and the like. Accordingly, the membrane modules and the frame body are made of different materials having different thermal expansion coefficients, and thus it is necessary to set the size of the frame body considering the variations in the ambient temperature.
In addition, when such a membrane separation device is submerged into the biological treatment tank, the volume of the membrane modules made of resin might be increased by absorbing water in the tank, and thus it is also necessary to set the volume of the metal frame body considering the expansion of the volume of the membrane module in advance.
Furthermore, when such a membrane separation device is transported by loading in a bulk container and if such a container transportation occurs under tropical regions right on the equator, for example, the temperature inside the container rises significantly, thereby causing such a phenomenon in which resin casings constituting the liquid collection units of the membrane modules expand under a high temperature and then shrink into a size smaller than the original size when the temperature returns to a room temperature, whereby the volume of the membrane modules tends to become smaller than the volume of the metal frame body.
Therefore, it is possible to set the size of the frame body considering the shrinking effect as described above, by performing a so-called annealing such that the membrane modules are exposed to a high temperature environment in advance so as to remove the molding distortion of the resin casings and the like. However, it is not practical since the manufacturing cost would increase remarkably, if the manufacture process should include such a process for exposing all of the membrane modules to a high temperature environment.
In such a situation, if the membrane separation device in which the volume of the membrane modules stacked in multiple stages in a frame body is smaller than that of the frame body is submerged and disposed in a biological treatment tank, and then a filtering operation is performed by aeration using a diffuser device provided below the membrane separation device, the membrane modules vibrate in a vertical direction within the frame body due to a upward flow caused by the aeration. The resulting wear and tear would cause problems of a considerably shortened life of the membrane modules.